Imbalance compensation device, transmission device, reception device, and imbalance compensation method

ABSTRACT

Provided is an imbalance compensation device that compensates for an imbalance between an in-phase component and a quadrature-phase component of a signal, the imbalance compensation device including: an extracting unit that extracts a signal component in an upper sideband or a signal component in a lower sideband from the signal; a measuring unit that measures power of the signal component in the upper sideband or the signal component in the lower sideband extracted by the extracting unit; and an adjusting unit that adjusts a parameter related to the imbalance, in accordance with the power measured by the measuring unit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2017-069235 filed on Mar. 30,2017, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the embodiments described herein relates to animbalance compensation device, a transmission device, a receptiondevice, and an imbalance compensation method.

BACKGROUND

In response to an increasing demand for large-volume data transmission,digital coherent optical transmission systems have been studied anddeveloped to enable transmission at 100 Gbps or faster with singlewavelength light, for example. Unlike an intensity modulation system, adigital coherent optical transmission system uses not only opticalintensities but also optical phases in signal modulation. An example ofsuch a modulation method is quadrature amplitude modulation (QAM). ByQAM, the amplitudes of respective signals of an in-phase component and aquadrature-phase component are adjusted.

In a modulation system of this type, a skew, a power difference, and aquadrature deviation (phase rotation) are generated between a signal ofan in-phase component and a signal of a quadrature-phase component ineach transmitter and each receiver. This phenomenon is called IQimbalance (also IQ unbalance, IQ incompleteness, or the like), andcauses signal deterioration. To counter this, Japanese PatentApplication Publication Nos. 2009-147498 and 2012-85302 (hereinafter,referred to as Patent Documents 1 and 2, respectively), for example,disclose IQ imbalance compensation means.

SUMMARY

According to an aspect of the embodiments, there is provided animbalance compensation device that compensates for an imbalance betweenan in-phase component and a quadrature-phase component of a signal, theimbalance compensation device including: an extracting unit thatextracts a signal component in an upper sideband or a signal componentin a lower sideband from the signal; a measuring unit that measurespower of the signal component in the upper sideband or the signalcomponent in the lower sideband extracted by the extracting unit; and anadjusting unit that adjusts a parameter related to the imbalance, inaccordance with the power measured by the measuring unit.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows graphs indicating example spectrums of signals.

FIG. 2 is a diagram showing an example configuration to be used in an IQimbalance numerical value simulation.

FIG. 3 shows waveform charts each indicating an example of changes inthe spectrum of a signal depending on an IQ imbalance.

FIG. 4 shows waveform charts each indicating an example of the spectrumof a signal that has passed through a bandpass filter.

FIG. 5 shows waveform charts each indicating an example of changes inthe power of a crosstalk component depending on the center frequency ofthe passband of a bandpass filter.

FIG. 6 is a configuration diagram showing an example of a transmissiondevice and an imbalance compensation device.

FIG. 7 is a flowchart showing an example of an IQ imbalance compensationprocess in the transmission device.

FIG. 8 is a flowchart showing another example of an IQ imbalancecompensation process in the transmission device.

FIG. 9 is a configuration diagram showing an example of a receptiondevice and an imbalance compensation device.

FIG. 10 is a graph showing an example of the center frequency of localoscillation light of the reception device.

FIG. 11 is a flowchart showing an example of an IQ imbalancecompensation process in the reception device.

FIG. 12 is a flowchart showing another example of an IQ imbalancecompensation process in the reception device.

FIG. 13 is a configuration diagram showing an example of a transmissionsystem that compensates for an IQ imbalance in a transmission device.

FIG. 14 is a configuration diagram showing another example of atransmission system that compensates for an IQ imbalance in atransmission device.

FIG. 15 is a configuration diagram showing another example of atransmission system that compensates for an IQ imbalance in atransmission device.

FIG. 16 is a flowchart showing an example of a control operation of atransmission system.

DESCRIPTION OF EMBODIMENTS

However, the technique disclosed in Patent Document 1 requires acomplicated process for estimating an IQ imbalance from the locations ofa reference signal and null symbols. The technique disclosed in PatentDocument 2 requires a calculation process based on complicatedmathematical formulas.

FIG. 1 shows graphs of example signal spectrums. Reference sign G1indicates crosstalk components Na and Nb generated in the upper sideband(USB) Sa and the lower sideband (LSB) Sb of a signal spectrum due to anIQ imbalance.

S _(α)(ω)=cos(ωτ/2)S(ω)+sin(ωτ/2)S*(−ω)  (1)

Where the spectrum of an ideal signal that does not have any skewbetween an in-phase component and a quadrature-phase component isrepresented by S(ω), the spectrum S_(α)(ω) having a skew τ existingtherein is expressed by the above equation (1). In the equation (1), thevariable ω is the angular velocity (=2π×frequency).

S _(β)(ω)=K ₁ S(ω)+K ₂ S*(−ω)  (2)

Meanwhile, the spectrum S_(β)(ω) of a signal having a power differenceand quadrature deviation existing between an in-phase component and aquadrature-phase component is expressed by the above equation (2) in thesame manner as the equation (1). In the equation (2), the variables K1and K2 are complex coefficients determined by the power difference andthe quadrature deviation.

As can be seen from the equation (1) and the equation (2), the IQimbalance expressed on a frequency axis is the crosstalk components Naand Nb, each of which is generated in one of the upper sideband Sa andthe lower sideband Sb of a signal by the other one of the upper sidebandSa and the lower sideband Sb. More specifically, as indicated by dashedlines, the upper sideband Sa interferes with the lower sideband Sb, togenerate the crosstalk component Na. The lower sideband Sb interfereswith the upper sideband Sa, to generate the crosstalk component Nb.

Therefore, as indicated by reference sign G2, in a case where a singlesideband signal having its signal band in the upper sideband Sa istransmitted/received, the crosstalk component Na is added to the lowersideband Sb of the signal. As a result, it becomes possible to detectthe magnitude of the IQ imbalance (or the degree of the IQ imbalance) bymeasuring the power of the crosstalk component Na.

Further, as indicated by reference sign G3, the magnitude of an IQimbalance can be detected by transmitting/receiving a signal having adifference in mean power value between the signal component in the uppersideband Sa and the signal component in the lower sideband Sb, insteadof a single sideband signal. More specifically, the power of the signalcomponent in the upper sideband Sa is greater than the mean power valueof the signal component in the lower sideband Sb by ΔP, and accordingly,the magnitude of the IQ imbalance can be detected from the power of thesignal component in the lower sideband Sb having the crosstalk componentNa added thereto.

Also, as indicated by reference sign G4, in a case where a singlesideband signal having its signal band in the lower sideband Sb istransmitted/received, the crosstalk component Nb is added to the uppersideband Sa of the signal. Because of this, the magnitude of an IQimbalance can be detected by measuring the power of the crosstalkcomponent Nb in the upper sideband Sa. Further, in a case where a signalin which the power of the signal component in the lower sideband Sb isgreater than the mean power value of the signal component in the lowersideband Sb by a predetermined value is transmitted/received, themagnitude of an IQ imbalance can be detected in the same manner asabove.

As a signal having a difference in mean power value between the signalcomponent in the upper sideband Sa and the signal component in the lowersideband Sb is transmitted/received in the above manner, the magnitudeof an IQ imbalance can be detected. Further, in a case where a singlesideband signal having its signal band in the upper sideband Sa or thelower sideband Sb is transmitted/received, the lower sideband Sb or theupper sideband Sa on the opposite side contains only the crosstalkcomponent Na or Nb. Thus, the magnitude of an IQ imbalance can bedetected with high precision.

FIG. 2 is a diagram showing an example configuration to be used in IQimbalance numerical value simulations. The configuration for numericalvalue simulations includes a Nyquist filter 90, low-pass filters (LPFs)91 and 98, adders 920 and 921, an amplifier 93, a Hilbert transform unit94, a skew adding unit 95, multipliers 96 and 97, and a monitor unit 99.The monitor unit 99 includes a bandpass filter (BPF) 990 and a powerdetecting unit 991.

The Nyquist filter 90, the LPF 91, the adder 920, the amplifier 93, theadder 921, the BPF 990, and the power detecting unit 991 are connectedin series in this order. The Hilbert transform unit 94, the skew addingunit 95, the multipliers 96 and 97, and the LPF 98 are connected inseries in this order.

As a signal S passes through the Nyquist filter 90, the waveform of thesignal S is shaped. After that, the signal S is divided, and the dividedsignals S are input to the LPF 91 and the Hilbert transform unit 94. Itshould be noted that the modulation method for the signal S is “SSBON-OFF keying”, the symbol rate is 32 (Gbaud), and the roll-off rate ofthe Nyquist filter 90 is 0.01.

Passing through the LPF 91, one of the signals S is transformed into asignal Si having an in-phase component of SSB. The other signal S issubjected to Hilbert transform by the Hilbert transform unit 94, andturns into a signal having only an SSB component. After the Hilberttransform, a skew τ (ps) is added to the signal S by the skew addingunit 95.

A quadrature deviation (per is then added to the signal S by themultiplier 96, and a power error according to a coefficient K is furtheradded to the signal S by the multiplier 97 in the latter stage. In thismanner, an IQ imbalance can be given to the signal S by the skew addingunit 95 and the multipliers 96 and 97.

After that, the signal S passes through the LPF 98, and is transformedinto a signal Sq of a quadrature-phase component of SSB. It should benoted that the LPFs 91 and 98 are Bessel functions having quartictransfer functions, for example, and each have a passband of 21 (GHz).

The signal Si of the in-phase component and the signal Sq of thequadrature-phase component are input to the adder 920, and are thencombined. The combined signal S′ contains only the signal component inthe upper sideband, as indicated by reference sign G2 in FIG. 1.

The signal S′ is amplified by the amplifier 93 so that the output powerreaches a predetermined value, and additive white gaussian noise (AWGN)is then added to the signal S′ by the adder 921. The additive whitegaussian noise is equivalent to the optical signal-to-noise ratio (OSNR)of 50 (dB), for example.

The monitor unit 99 detects the power of the crosstalk component of thesignal S′. The BPF 990 passes the signal component in the passband fromthe signal S′. Reference sign G5 indicates the spectrum of the signal S′and the passband B of the BPF 990. The center frequency fc of thepassband B deviates from the center frequency fo of the signal S′ towardthe lower sideband by Δf. Because of this, the BPF 990 extracts thecrosstalk component Nc of the lower sideband of the signal S′, andoutputs the crosstalk component Nc to the power detecting unit 991. Itshould be noted that the BPF 990 is a gaussian function having a quartictransfer function, and has a passband of 12.5 (GHz).

The power detecting unit 991 detects the power of the crosstalkcomponent Nc of the signal S′. As will be described later, the magnitudeof an IQ imbalance can be measured from the power of a crosstalkcomponent N.

FIG. 3 shows waveform charts showing example changes in the spectrum ofa signal S′ depending on an IQ imbalance. Each waveform chart shows thewaveform of a signal S′ to be input to the BPF 990. In each waveformchart, the abscissa axis indicate frequency, and the ordinate axisindicates power (dB). The center frequency of each signal S′ is 0 (GHz).

Reference sign Ga indicates changes in the spectrum when the skewbetween the in-phase component and the quadrature-phase component of thesignal S′ is changed from 0 (ps) to 1 (ps) to 3 (ps) to 5 (ps). As theskew increases, the power of the crosstalk component Nc of the lowersideband (−50 to 0 (GHz)) also increases.

Reference sign Gb indicates changes in the spectrum when the powerdifference between the in-phase component and the quadrature-phasecomponent of the signal S′ is changed from 0(%) to 10(%) to 20(%) to30(%). As the power difference increases, the power of the crosstalkcomponent Nc of the lower sideband also increases.

Reference sign Gc indicates changes in the spectrum when the quadraturedeviation between the in-phase component and the quadrature-phasecomponent of the signal S′ is changed from 0 (rad) to 0.1 (rad) to 0.2(rad) to 0.3 (rad). As the quadrature deviation increases, the power ofthe crosstalk component Nc of the lower sideband also increases.

As described above, the power of the crosstalk component Nc increaseswith the magnitude of the IQ imbalance. Therefore, the crosstalkcomponent Nc of the lower sideband of the signal S′ is extracted by theBPF 990, and the power is detected by the power detecting unit 991, sothat the magnitude of the IQ imbalance can be monitored.

FIG. 4 shows waveform charts showing example spectrums of a signal S′that has passed through the BPF 990. Reference sign Gd indicates thespectrum of the signal S′ before and after the signal S′ passes throughthe BPF 990 in a case where the skew is 3 (ps). In this manner, the BPF990 can extract the crosstalk component Nc from the signal S′.

Reference sign Gf indicates changes in the spectrum when the differenceΔf between the center frequency fo of the signal S′ and the centerfrequency fc of the passband B is set at 0 (GHz), 4 (GHz), and 12 (GHz)in a case where the skew is 3 (ps). In this example, in a case where thedifference Δf is 12 (GHz), the crosstalk component Nc can be extractedin a most preferred manner.

FIG. 5 is a waveform chart showing example changes in the power of thecrosstalk component Nc depending on the center frequency fc of thepassband of the BPF 990. Reference sign Gg indicates the changes in thepower with respect to the skew when the difference Δf is set at 0 (GHz),4 (GHz), 8 (GHz), 12 (GHz), 16 (GHz), and 20 (GHz). Reference sign Ghindicates the changes in the power with respect to the power differencewhen the difference Δf is set at 0 (GHz), 4 (GHz), 8 (GHz), 12 (GHz), 16(GHz), and 20 (GHz). Reference sign Gi indicates the changes in thepower with respect to the quadrature deviation when the difference Δf isset at 0 (GHz), 4 (GHz), 8 (GHz), 12 (GHz), 16 (GHz), and 20 (GHz).

As can be seen from the graphs indicated by reference signs Gg throughGi, in the cases where the difference Δf is 0 (GHz) or 4 (GHz), thecomponent in the upper sideband of the signal S′ is stronger, andtherefore, IQ imbalance monitoring cannot be performed. However, in thecases where the difference Δf is 8 (GHz), 12 (GHz), 16 (GHz), or 20(GHz), the component in the lower sideband of the signal S′ is strong,and therefore, IQ imbalance monitoring can be performed.

Therefore, the imbalance compensation device of the embodimentcompensates for an IQ imbalance by adjusting the power difference, theskew, and the quadrature deviation between the in-phase component andthe quadrature-phase component, in accordance with the results of IQimbalance monitoring.

FIG. 6 is a configuration diagram showing an example of a transmissiondevice 1 and an imbalance compensation device 3. The imbalancecompensation device 3 monitors the IQ imbalance in the transmissiondevice 1 from a test signal St transmitted from the transmission device1, and, for the transmission device 1, adjusts the power difference, theskew, and the quadrature deviation between the in-phase component andthe quadrature-phase component of the test signal St, in accordance withthe results of the monitoring. The test signal St is an example of asignal containing an in-phase component and a quadrature-phasecomponent.

The transmission device 1 transmits the test signal St according to adigital coherent optical transmission method, using a polarizationmultiplexing technique. The transmission device 1 includes atransmission processing circuit 10, BPFs 18 a through 18 d,digital-to-analog converters 12 a through 12 d, and an opticalmodulating unit 19. The optical modulating unit 19 includes a lightsource 11, Mach-Zehnder modulators (MZMs) 13 a through 13 d, apolarization beam splitter (PBS) 14, a polarization beam combiner (PBC)15, and phase shifters 16 a and 16 b.

The transmission processing circuit 10 generates four digital signalsXi, Xq, Yi, and Yq from a data signal Dt input from another device. Thetransmission processing circuit 10 includes a distributing unit 100,mapping units 101 a and 101 b, amplitude adjusting units 102 a through102 d, and deskewing units 103 a through 103 d. The transmissionprocessing circuit 10 may be a digital signal processor (DSP), forexample. However, the transmission processing circuit 10 is notnecessarily a DSP, but may be a field programmable gate array (FPGA),for example. Further, the transmission processing circuit 10 may includefunctions other than the above.

The distributing unit 100 distributes the data signal Dt to the mappingunits 101 a and 101 b. The mapping units 101 a and 101 b map the datasignal Dt as symbols for a modulation process (such as QAM). The mappingunit 101 a performs a mapping process on the data signal Dt to beassigned to X-polarization. The mapping unit 101 b performs a mappingprocess on the data signal Dt to be assigned to Y-polarization.

The mapping unit 101 a outputs the in-phase component of the data signalDt of the X-polarization to the amplitude adjusting unit 102 a, andoutputs the quadrature-phase component to the amplitude adjusting unit102 b. The mapping unit 101 b outputs the in-phase component of the datasignal Dt of the Y-polarization to the amplitude adjusting unit 102 c,and outputs the quadrature-phase component to the amplitude adjustingunit 102 d.

The amplitude adjusting units 102 a through 102 d each include anamplifying circuit and the like, and adjust the amplitudes of the signalcomponents input from the mapping units 101 a and 101 b, in accordancewith a set value supplied from the imbalance compensation device 3. Thesignal components subjected to the amplitude adjustment are input to thedeskewing units 103 a through 103 d. It should be noted that theamplitude adjusting units 102 a through 102 d may be provided in a stagelater than the deskewing units 103 a through 103 d.

The deskewing units 103 a through 103 d each include a delay insertingcircuit and the like, and adjust the skews of the signal componentsinput from the amplitude adjusting units 102 a through 102 d, inaccordance with a set value supplied from the imbalance compensationdevice 3. The signal components subjected to the skew adjustment areinput to the BPFs 18 a through 18 d.

The BPFs 18 a through 18 d are electrical filters, and extract thesignal components in a predetermined passband from the signal componentsinput from the deskewing units 103 a through 103 d. More specifically,the BPFs 18 a through 18 d extract signal components in a predeterminedpassband, to generate an SSB signal like the one indicated by referencesign G2 in FIG. 1.

Alternatively, the BPFs 18 a through 18 d do not necessarily generate anSSB signal, but may generate a signal having a difference between themean power value of the signal component in the upper sideband and themean power value of the signal component in the lower sideband, asindicated by reference sign G3 in FIG. 1. That is, the BPFs 18 a through18 d are examples of the converting unit, and convert a signal so thatthe power of the signal component in the upper sideband of the signaland the power of the signal component in the lower sideband of thesignal differ from each other.

The BPF 18 a outputs the extracted signal as the digital signal Xi ofthe in-phase component of the X-polarization to the DAC 12 a, and theBPF 18 b outputs the extracted signal as the digital signal Xq of thequadrature-phase component of the X-polarization to the DAC 12 b. TheBPF 18 c outputs the extracted signal as the digital signal Yi of thein-phase component of the Y-polarization to the DAC 12 c, and the BPF 18d outputs the extracted signal as the digital signal Yq of thequadrature-phase component of the Y-polarization to the DAC 12 d.

The DACs 12 a through 12 d convert the digital signals Xi, Xq, Yi, andYq into analog signals. The analog signals are input to the MZMs 13 athrough 13 d. It should be noted that the DACs 12 a through 12 d may beformed in the transmission processing circuit 10. The amplitudeadjusting units 102 a through 102 d and the deskewing units 103 athrough 103 d are provided in stages earlier than the DAC 12 a through12 d, but may be provided in stages later than the DACs 12 a through 12d.

The light source 11 is a laser diode (LD), for example, and outputslocal oscillation light LOs at a predetermined frequency to the PBS 14.The PBS 14 divides the local oscillation light LOs between the X-axisand the Y-axis (polarizing axes). The X-axis component of the localoscillation light LOs is input to each of the MZMs 13 a and 13 b, andthe Y-axis component of the local oscillation light LOs is input to eachof the MZMs 13 c and 13 d.

The MZMs 13 a through 13 d optically modulate the local oscillationlight LOs in accordance with the analog signals from the DACs 12 athrough 12 d. More specifically, the MZMs 13 a and 13 b opticallymodulate the X-axis component of the local oscillation light LOs inaccordance with the analog signals from the DACs 12 a and 12 b, and theMZMs 13 c and 13 d optically modulate the Y-axis component of the localoscillation light LOs in accordance with the analog signals from theDACs 12 c and 12 d.

The phase shifters 16 a and 16 b are connected to the stages after theMZMs 13 b and 13 d corresponding to the quadrature-phase components (Xq,Yq). The phase shifters 16 a and 16 b are formed with phase modulatorsor the like, and shift the phases of light output from the MZMs 13 b and13 d by π/2. The phase shifters 16 a and 16 b also adjust the phaseshift amount (π/2) in accordance with a set value supplied from theimbalance compensation device 3. Accordingly, the phase shift amount isa value obtained by adding a fine adjustment value based on the setvalue to π/2.

The X-axis component and the Y-axis component of the optically-modulatedlocal oscillation light LOs are input to the PBC 15. The PBC 15 performspolarization combining of the X-axis component and the Y-axis componentof the local oscillation light LOs, and outputs the resultant signal asthe test signal St to the imbalance compensation device 3.

The imbalance compensation device 3 includes a BPF 30, a power measuringunit 31, and a parameter adjusting unit 32. The imbalance compensationdevice 3 compensates for the IQ imbalance in the transmission device 1with respect to each kind of polarization.

Therefore, in a case where the IQ imbalance is to be compensated forwith respect to the X-polarization, the parameter adjusting unit 32 putsthe digital signals Yi and Yq of the Y-polarization into an unmodulatedstate by controlling and stopping the operations of the MZMs 13 c and 13d of the Y-polarization. Likewise, in a case where the IQ imbalance isto be compensated for with respect to the Y-polarization, the parameteradjusting unit 32 puts the digital signals Xi and Xq of theX-polarization into an unmodulated state by controlling and stopping theoperations of the MZMs 13 a and 13 b of the X-polarization. It should benoted that the parameter adjusting unit 32 may switching on and off anoptical switch, for example, to perform control so that only one of theX-axis component and the Y-axis component of the local oscillation lightLOs is input to the MZMs 13 a through 13 d.

The BPF 30 is an optical filter. The BPF 30 receives the test signal Stinput from the PBC 15, and passes the signal component in the uppersideband or the lower sideband of the test signal St. That is, the BPF30 is an example of the extracting unit, and extracts the signalcomponent in the upper sideband or the lower sideband from the testsignal St.

For example, in a case where the test signal St indicated by referencesigns G2 and G3 in FIG. 1 is input to the BPF 30, the BPF 30 extractsthe signal component in the lower sideband Sb. In a case where the testsignal St indicated by reference sign G4 in FIG. 1 is input to the BPF30, the BPF 30 extracts the signal component in the upper sideband Sa.That is, the BPF 30 extracts the signal component in the upper sidebandSa or the signal component in the lower sideband Sb, whichever has thesmaller power (mean value).

As a result, the crosstalk component generated in the test signal St dueto the IQ imbalance is extracted by the BPF 30. The signal componentextracted by the BPF 30 is input to the power measuring unit 31.

The power measuring unit 31 is formed with a photodiode, for example,and measures the power of the signal component extracted by the BPF 30.Thus, the imbalance compensation device 3 can monitor the magnitude ofthe IQ imbalance in the transmission device 1, in accordance with thepower measured by the power measuring unit 31. The power measuring unit31 notifies the parameter adjusting unit 32 of the measured power. Itshould be noted that the power measuring unit 31 is an example of themeasuring unit.

The parameter adjusting unit 32 is an example of the adjusting unit, andadjusts the parameters related to the IQ imbalance, in accordance withthe power measured by the power measuring unit 31. The parameteradjusting unit 32 is formed with an FPGA, for example, and adjusts therespective set values of the amplitude adjusting units 102 a through 102d, the deskewing units 103 a through 103 d, and the phase shifters 16 aand 16 b. The respective set values are an example of the parametersrelated to the IQ imbalance.

As described above, the imbalance compensation device 3 measures thepower of the test signal St having a difference between the mean powervalue of the signal component in the upper sideband and the mean powervalue of the signal component in the lower sideband, and, in accordancewith the power, compensates for the imbalance between the in-phasecomponent and the quadrature-phase component, or the IQ imbalance, ofthe test signal St. Thus, the imbalance compensation device 3 canreadily compensate for the IQ imbalance.

More specifically, the parameter adjusting unit 32 adjusts therespective set values of the amplitude adjusting units 102 a through 102d, the deskewing units 103 a through 103 d, and the phase shifters 16 aand 16 b so that the power is minimized. Accordingly, the imbalancecompensation device 3 can minimize the magnitude of the IQ imbalance byminimizing the crosstalk component.

Although the imbalance compensation device 3 is provided independentlyof the transmission device 1 in this example, the imbalance compensationdevice 3 may be provided inside the transmission device 1. In that case,the transmission device 1 further includes the BPF 30, the powermeasuring unit 31, and the parameter adjusting unit 32. Thus, thetransmission device 1 can achieve the same effects as those describedabove.

FIG. 7 is a flowchart showing an example of the IQ imbalancecompensation process in the transmission device 1. The parameteradjusting unit 32 stops the optical modulation of one of thepolarization components of the X-axis and the Y-axis (step SU). In acase where the IQ imbalance is to be compensated for with respect to theX-polarization, the parameter adjusting unit 32 in this step controlsand stops the operations of the MZMs 13 c and 13 d of theY-polarization. In a case where the IQ imbalance is to be compensatedfor with respect to the Y-polarization, the parameter adjusting unit 32in this step controls and stops the operations of the MZMs 13 a and 13 bof the X-polarization.

In the procedures thereafter, only the IQ imbalance compensation processwith respect to either the X-polarization or the Y-polarization isperformed, but the same process is also performed with respect to theother polarization. In a case where the IQ imbalance is compensated forwith respect to the X-polarization, the parameter adjusting unit 32adjusts the respective set values of the amplitude adjusting units 102 aand 102 b, the deskewing units 103 a and 103 b, and the phase shifter 16a.

Likewise, in a case where the IQ imbalance is compensated for withrespect to the Y-polarization, the parameter adjusting unit 32 adjuststhe respective set values of the amplitude adjusting units 102 c and 102d, the deskewing units 103 c and 103 d, and the phase shifter 16 b. Inthe description below, however, the respective adjusted values of theamplitude adjusting units 102 a through 102 d, the deskewing units 103 athrough 103 d, and the phase shifters 16 a and 16 b will be collectivelydescribed as the adjustment targets.

The BPFs 18 a through 18 d generate an SSB signal by extracting thesignal component in the upper sideband or the lower sideband from thedata signal Dt (step St2). An SSB signal having its signal band in theupper sideband in this example. However, the SSB signal does notnecessarily have the signal band in the upper sideband, but may have itssignal band in the lower sideband.

The BPFs 18 a through 18 d do not necessarily generate an SSB signal,but may generate a signal having a difference between the mean powervalue of the signal component in the upper sideband and the mean powervalue of the signal component in the lower sideband. That is, the BPFs18 a through 18 d convert the transmission target signal into a signalhaving a difference between the mean power value of the signal componentin the upper sideband and the mean power value of the signal componentin the lower sideband.

The BPF 30 then extracts the signal component in the lower sideband (LSBcomponent) from the test signal St (step St3). As a result, thecrosstalk component generated in the test signal St due to the IQimbalance in the transmission device 1 is extracted. However, in a casewhere the SSB signal generated in step St2 has the signal band in thelower sideband, the BPF 30 extracts the signal component in the uppersideband (USB component) from the test signal St.

The parameter adjusting unit 32 then initializes the respective setvalues of the amplitude adjusting units 102 a through 102 d, theamplitude adjusting units 102 a through 102 d, the deskewing units 103 athrough 103 d, and the phase shifters 16 a and 16 b (step St4). In theprocedures thereafter, the parameter adjusting unit 32 sequentiallyadjusts the respective set values of the deskewing units 103 a through103 d and the phase shifters 16 a and 16 b. However, the adjustment isnot limited to this order, and only one of the set values may beadjusted. In the description below, the respective set values of theamplitude adjusting units 102 a through 102 d, the deskewing units 103 athrough 103 d, and the phase shifters 16 a and 16 b will be referred toas “the amplitude set value”, “the skew set value”, and “the phase shiftamount set value”.

The power measuring unit 31 measures the power of the signal componentin the lower sideband extracted by the BPF 30 (step St5). The measuredpower (measured value) is reported to the parameter adjusting unit 32,and is stored into a storage such as a memory in the parameter adjustingunit 32.

The parameter adjusting unit 32 then changes the amplitude set value(step St6). For example, the parameter adjusting unit 32 increases theset value by a predetermined value. The parameter adjusting unit 32 thendetermines whether the set value is within a predetermined adjustmentrange (step St7). The set value adjustment range is determined from thefunctions of the amplitude adjusting units 102 a through 102 d, forexample.

If the set value is within the adjustment range (Yes in step St7), theprocedures in step St5 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St7), theparameter adjusting unit 32 sets such a set value in the amplitudeadjusting units 102 a through 102 d as to minimize the power of thesignal component in the lower sideband (step St8). With this, theimbalance compensation device 3 can minimize the power differencebetween the in-phase component and the quadrature-phase component.

The power measuring unit 31 then measures the power of the signalcomponent in the lower sideband extracted by the BPF 30 (step St9). Themeasured power is reported to the parameter adjusting unit 32, and isstored into a storage such as a memory in the parameter adjusting unit32.

The parameter adjusting unit 32 then changes the skew set value (stepSt10). For example, the parameter adjusting unit 32 increases the setvalue by a predetermined value. The parameter adjusting unit 32 thendetermines whether the set value is within a predetermined adjustmentrange (step St11). The set value adjustment range is determined from thefunctions of the deskewing units 103 a through 103 d, for example.

If the set value is within the adjustment range (Yes in step St11), theprocedures in step St9 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St11), theparameter adjusting unit 32 sets such a set value in the deskewing units103 a through 103 d as to minimize the power of the signal component inthe lower sideband (step St12). With this, the imbalance compensationdevice 3 can minimize the skew between the in-phase component and thequadrature-phase component.

The power measuring unit 31 then measures the power of the signalcomponent in the lower sideband extracted by the BPF 30 (step St13). Themeasured power is reported to the parameter adjusting unit 32, and isstored into a storage such as a memory in the parameter adjusting unit32.

The parameter adjusting unit 32 then changes the phase shift amount setvalue (step St14). For example, the parameter adjusting unit 32increases the set value by a predetermined value. The parameteradjusting unit 32 then determines whether the set value is within apredetermined adjustment range (step St15). The set value adjustmentrange is determined from the functions of the phase shifters 16 a and 16b, for example.

If the set value is within the adjustment range (Yes in step St15), theprocedures in step St13 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St15), theparameter adjusting unit 32 sets such a set value in the phase shifters16 a and 16 b as to minimize the power of the signal component in thelower sideband (step St16). With this, the imbalance compensation device3 can minimize the quadrature deviation between the in-phase componentand the quadrature-phase component.

In this manner, the IQ imbalance compensation process is performed. Animbalance compensation method based on this compensation process canachieve the same effects as those of the imbalance compensation device3.

In this example, the BPF 30 extracts the crosstalk component from thetest signal St. However, the BPF 30 does not necessarily extract thecrosstalk component, but may extract a signal component of the source ofthe crosstalk component. For example, in a case where the test signal Sthas the spectrum indicated by reference sign G2 in FIG. 1, the BPF 30extracts the signal component in the upper sideband Sa. In a case wherethe test signal St has the spectrum indicated by reference sign G4 inFIG. 1, the BPF 30 extracts the signal component in the lower sidebandSb.

That is, the BPF 30 extracts the signal component in the upper sidebandSa or the signal component in the lower sideband Sb, whichever has thegreater power (mean value). Where the crosstalk component is larger, theextracted signal component has a smaller power. Therefore, the parameteradjusting unit 32 adjusts the respective set values of the amplitudeadjusting units 102 a through 102 d, the deskewing units 103 a through103 d, and the phase shifters 16 a and 16 b so that the power measuredby the power measuring unit 31 is maximized. The compensation process inthis case is described below.

FIG. 8 is a flowchart showing another example of the IQ imbalancecompensation process in the transmission device 1. In FIG. 8, the sameprocedures as those shown in FIG. 7 are denoted by the same referencesigns as those used in FIG. 7, and explanation thereof will not berepeated.

The BPFs 18 a through 18 d generate an SSB signal having its signal bandin the lower sideband (step St2 a). The BPF 30 then extracts the signalcomponent in the upper sideband (USB component), which is the cross-talkcomponent, from the test signal St (step St3 a).

The power measuring unit 31 measures the power of the signal componentin the upper sideband (USB component) (steps St5 a, St9 a, and St13 a).The parameter adjusting unit 32 sets the respective set values so thatthe measured power is maximized (steps St8 a, St12 a, and St16 a).

Thus, the imbalance compensation device 3 can minimize the powerdifference, the skew, and the quadrature deviation between the in-phasecomponent and the quadrature-phase component, by minimizing thecrosstalk component.

Next, a case where an IQ imbalance in a reception device is compensatedfor is described.

FIG. 9 is a configuration diagram showing an example of a receptiondevice 2 and an imbalance compensation device 4. The reception device 2receives a test signal Sr containing an in-phase component and aquadrature-phase component. The imbalance compensation device 4compensates for an IQ imbalance in the reception device 2. The testsignal Sr has a different power at each frequency in the band in whichthe reception device 2 can perform reception.

The reception device 2 includes a reception processing circuit 20,analog-to-digital converters (ADCs) 22 a through 22 d, and a coherentreceiver 29. The coherent receiver 29 includes a light source 21,photodiodes (PDs) 23 a through 23 d, 90-degree optical hybrid circuits240 and 241, and PBSs 25 and 26.

The PBS 26 divides an input test signal Sr into an X-axis component anda Y-axis component, and outputs the X-axis component and the Y-axiscomponent to the 90-degree optical hybrid circuits 240 and 241,respectively. The light source 21 inputs local oscillation light LOr tothe PBS 25. The PBS 25 divides the local oscillation light LOr into anX-axis component and a Y-axis component, and outputs the X-axiscomponent and the Y-axis component to the 90-degree optical hybridcircuits 240 and 241, respectively.

The 90-degree optical hybrid circuit 240 has a waveguide for causing theX-axis component of the test signal Sr and the X-axis component of thelocal oscillation light LOr to interfere with each other, and detectsthe X-axis component of the test signal Sr. As a result of thedetection, the 90-degree optical hybrid circuit 240 outputs opticalcomponents corresponding to the amplitudes and the phases of thein-phase component and the quadrature-phase component to the PDs 23 aand 23 b, respectively.

The 90-degree optical hybrid circuit 241 has a waveguide for causing theY-axis component of the test signal Sr and the Y-axis component of thelocal oscillation light LOr to interfere with each other, and detectsthe Y-axis component of the test signal Sr. As a result of thedetection, the 90-degree optical hybrid circuit 241 outputs opticalcomponents corresponding to the amplitudes and the phases of thein-phase component and the quadrature-phase component to the PDs 23 cand 23 d, respectively.

The PDs 23 a through 23 d convert the input optical components intoelectrical signals, and output the electrical signals to the ADCs 22 athrough 22 d, respectively. The ADCs 22 a through 22 d convert theelectrical signals input from the PDs 23 a through 23 d into digitalsignals Xi, Xq, Yi, and Yq, respectively. The digital signals Xi, Xq,Yi, and Yq are input to the reception processing circuit 20.

The center frequency of the local oscillation light LOr of the lightsource 21 is set at a different value from the center frequency of thetest signal Sr by the imbalance compensation device 4. Thus, the testsignal Sr is converted into an SSB signal.

FIG. 10 is a graph showing an example of the center frequency fr of thelocal oscillation light LOr of the reception device 2. The centerfrequency fr of the local oscillation light LOr is set at the frequencyof the edge of the lower sideband of the spectrum of the test signal Sr,instead of the center frequency of the test signal Sr. As a result, thetest signal Sr becomes an SSB signal having its signal band in the uppersideband. However, the test signal Sr may be an SSB signal having itssignal band in the lower sideband. Further, the test signal Sr is notnecessarily an SSB signal, but may be a signal having a differencebetween the power of the signal component in the upper sideband and thepower of the signal component in the lower sideband.

In this manner, the light source 21 as an example of the converting unitconverts the test signal S so that the power (mean value) of the signalcomponent in the upper sideband of the test signal Sr and the power(mean value) of the signal component in the lower sideband of the testsignal Sr differ from each other. Therefore, the transmission device 1does not need to convert the test signal Sr. Thus, the imbalancecompensation device 4 can compensate for the IQ imbalance, using thesignal component extracted from the converted test signal Sr asdescribed later.

The reception processing circuit 20 includes amplitude adjusting units200 a through 200 d, deskewing units 201 a through 201 d, quadraturedeviation adjusting units 202 a through 202 d, and a demodulationprocessing unit 203. The reception processing circuit 20 may be a DSP,for example. However, the reception processing circuit 20 is notnecessarily a DSP, but may be an FPGA, for example. Further, thereception processing circuit 20 may include functions other than theabove.

The amplitude adjusting units 200 a through 200 d each include anamplifying circuit and the like, and adjust the amplitudes of thedigital signals Xi, Xq, Yi, and Yq input from the ADCs 22 a through 22d, in accordance with a set value supplied from the imbalancecompensation device 4. The digital signals Xi, Xq, Yi, and Yq subjectedto the amplitude adjustment are input to the deskewing units 201 athrough 201 d.

The deskewing units 201 a through 201 d each include a delay insertingcircuit and the like, and adjust the skews of the digital signals Xi,Xq, Yi, and Yq input from the amplitude adjusting units 200 a through200 d, in accordance with a set value supplied from the imbalancecompensation device 4. The digital signals Xi, Xq, Yi, and Yq subjectedto the skew adjustment are input to the demodulation processing unit203. The demodulation processing unit 203 performs a demodulationprocess on the digital signals Xi, Xq, Yi, and Yq.

It should be noted that the amplitude adjusting units 200 a through 200d may be provided in a stage later than the deskewing units 201 athrough 201 d. The amplitude adjusting units 200 a through 200 d and thedeskewing units 201 a through 201 d are provided in stages later thanthe ADCs 22 a through 22 d in this example, but may be provided instages earlier than the ADCs 22 a through 22 d. Although the digitalsignals Xi, Xq, Yi, and Yq from a stage later than the ADC 22 a through22 d are input to the imbalance compensation device 4 in this example,analog signals from a stage earlier than the ADCs 22 a through 22 d maybe input to the imbalance compensation device 4. In such a case,however, the amplitude adjusting units 200 a through 200 d and thedeskewing units 201 a through 201 d are provided in stages earlier thanthe ADCs 22 a through 22 d.

The imbalance compensation device 4 includes synthesis processing units40 a and 40 b, BPFs 41 a and 41 b that are electrical filters, powermeasuring units 42 a and 42 b, and a parameter adjusting unit 43. Thesynthesis processing unit 40 a, the BPF 41 a, and the power measuringunit 42 a are used in an IQ imbalance compensation process with respectto the digital signals Yi and Yq of Y-polarization, and the synthesisprocessing unit 40 b, the BPF 41 b, and the power measuring unit 42 bare used in an IQ imbalance compensation process with respect to thedigital signals Xi and Xq of X-polarization.

The synthesis processing unit 40 a includes a circuit such as an adder,and generates a signal Sy of Y-polarization from the digital signals Yiand Yq. The signal Sy is input to the BPF 41 a.

The BPF 41 a is an example of the extracting unit, and extracts thesignal component in the upper sideband or the signal component in thelower sideband from the signal Sy. In a case where the signal Sy is anSSB signal having its signal band in the upper sideband, the BPF 41 aextracts the signal component in the lower sideband of the signal Sy. Ina case where the signal Sy is an SSB signal having its signal band inthe lower sideband, the BPF 41 a extracts the signal component in theupper sideband of the signal Sy. As a result, the crosstalk componentgenerated in the signal Sy due to an IQ imbalance is extracted. Thesignal component extracted by the BPF 41 a is input to the powermeasuring unit 42 a.

The power measuring unit 42 a is an example of the measuring unit, andmeasures the power of the signal component extracted by the BPF 41 a.More specifically, the power measuring unit 42 a measures the power ofthe crosstalk component of the signal Sy. The measured power is reportedto the parameter adjusting unit 43.

The synthesis processing unit 40 b includes a circuit such as an adder,and generates a signal Sz of X-polarization from the digital signals Xiand Xq. The signal Sx is input to the BPF 41 b.

The BPF 41 b is an example of the extracting unit, and extracts thesignal component in the upper sideband or the signal component in thelower sideband from the signal Sx. In a case where the signal Sx is anSSB signal having its signal band in the upper sideband, the BPF 41 bextracts the signal component in the lower sideband of the signal Sx. Ina case where the signal Sx is an SSB signal having its signal band inthe lower sideband, the BPF 41 b extracts the signal component in theupper sideband of the signal Sx. As a result, the crosstalk componentgenerated in the signal Sx due to the IQ imbalance is extracted. Thesignal component extracted by the BPF 41 b is input to the powermeasuring unit 42 b.

The power measuring unit 42 b is an example of the measuring unit, andmeasures the power of the signal component extracted by the BPF 41 b.More specifically, the power measuring unit 42 b measures the power ofthe crosstalk component of the signal Sx. The measured power is reportedto the parameter adjusting unit 43.

The parameter adjusting unit 43 is an example of the adjusting unit, andadjusts the parameters related to the IQ imbalance, in accordance withthe powers reported from the power measuring units 42 a and 42 b. Morespecifically, with respect to the X-polarization, the parameteradjusting unit 43 adjusts the respective set values of the amplitudeadjusting units 200 a and 200 b, the deskewing units 201 a and 201 b,and the quadrature deviation adjusting units 202 a and 202 b, inaccordance with the powers reported from the power measuring unit 42 b.

With respect to the Y-polarization, the parameter adjusting unit 43adjusts the respective set values of the amplitude adjusting units 200 cand 200 d, the deskewing units 201 c and 201 d, and the quadraturedeviation adjusting units 202 c and 202 d, in accordance with the powersreported from the power measuring unit 42 a. It should be noted that therespective set values are an example of the parameters related to the IQimbalance.

In this manner, the imbalance compensation device 4 measures the powersof the signals Sx and Sy each having a difference between the mean powervalue of the signal component in the upper sideband and the mean powervalue of the signal component in the lower sideband, and, in accordancewith the powers, compensates for the imbalance between the in-phasecomponent and the quadrature-phase component, or the IQ imbalance, ofthe test signal Sr. Thus, the imbalance compensation device 4 canreadily compensate for the IQ imbalance.

More specifically, the parameter adjusting unit 43 adjusts therespective set values of the amplitude adjusting units 200 a through 200d, the deskewing units 201 a through 201 d, and the quadrature deviationadjusting units 202 a through 202 d so that the power is minimized.Accordingly, the imbalance compensation device 4 can minimize themagnitude of the IQ imbalance by minimizing the crosstalk component.

Although the imbalance compensation device 4 is provided independentlyof the reception device 2 in this example, the imbalance compensationdevice 4 may be provided inside the reception device 2. In that case,the reception device 2 further includes the synthesis processing units40 a and 40 b, the BPFs 41 a and 41 b, the power measuring units 42 aand 42 b, and the parameter adjusting unit 43. Thus, the receptiondevice 2 can achieve the same effects as those described above.

FIG. 11 is a flowchart showing an example of the IQ imbalancecompensation process in the reception device 2. In this example, onlythe compensation process with respect to the X-polarization or theY-polarization is described. In the description below, however, therespective adjustment values of the amplitude adjusting units 200 athrough 200 d, the deskewing units 201 a through 201 d, and thequadrature deviation adjusting units 202 a through 202 d will becollectively described as the adjustment targets.

The parameter adjusting unit 43 adjusts the center frequency fr of thelocal oscillation light LOr so that the test signal Sr is converted intoan SSB signal having its signal band in the upper sideband as shown inFIG. 10 (step St21). The BPFs 41 a and 41 b then extract the signalcomponent in the lower sideband (LSB component) from the signals Sx andSy obtained from the test signal Sr (step St22). As a result, thecrosstalk component generated in the test signal Sr due to the IQimbalance in the reception device 2 is extracted. In a case where theSSB signal generated in step St 21 has its signal band in the lowersideband, the BPFs 41 a and 41 b extract the signal component in theupper sideband (USB component) from the test signal Sr.

The parameter adjusting unit 43 then initializes the respective setvalues of the amplitude adjusting units 200 a through 200 d, thedeskewing units 201 a through 201 d, and the quadrature deviationadjusting units 202 a through 202 d (step St23). In the proceduresthereafter, the parameter adjusting unit 43 sequentially adjusts therespective set values of the amplitude adjusting units 200 a through 200d, the deskewing units 201 a through 201 d, and the quadrature deviationadjusting units 202 a through 202 d. However, the adjustment is notlimited to this order, and only one of the set values may be adjusted.In the description below, the respective set values of the amplitudeadjusting units 200 a through 200 d, the deskewing units 201 a through201 d, and the quadrature deviation adjusting units 202 a through 202 dwill be referred to as “the amplitude set value”, “the skew set value”,and “the quadrature deviation set value”.

The power measuring units 42 a and 42 b measure the power of the signalcomponent in the lower sideband extracted by the BPFs 41 a and 41 b(step St24). The measured power (measured value) is reported to theparameter adjusting unit 43, and is stored into a storage such as amemory in the parameter adjusting unit 43.

The parameter adjusting unit 43 then changes the amplitude set value(step St25). For example, the parameter adjusting unit 43 increases theset value by a predetermined value. The parameter adjusting unit 43 thendetermines whether the set value is within a predetermined adjustmentrange (step St26). The set value adjustment range is determined from thefunctions of the amplitude adjusting units 200 a through 200 d, forexample.

If the set value is within the adjustment range (Yes in step St26), theprocedures in step St24 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St26), theparameter adjusting unit 43 sets such a set value in the amplitudeadjusting units 200 a through 200 d as to minimize the power of thesignal component in the lower sideband (step St27). With this, theimbalance compensation device 4 can minimize the power differencebetween the in-phase component and the quadrature-phase component.

The power measuring units 42 a and 42 b then measure the power of thesignal component in the lower sideband extracted by the BPFs 41 a and 41b (step St28). The measured power is reported to the parameter adjustingunit 43, and is stored into a storage such as a memory in the parameteradjusting unit 43.

The parameter adjusting unit 32 then changes the skew set value (stepSt29). For example, the parameter adjusting unit 43 increases the setvalue by a predetermined value. The parameter adjusting unit 43 thendetermines whether the set value is within a predetermined adjustmentrange (step St30). The set value adjustment range is determined from thefunctions of the deskewing units 201 a through 201 d, for example.

If the set value is within the adjustment range (Yes in step St30), theprocedures in step St28 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St30), theparameter adjusting unit 43 sets such a set value in the deskewing units201 a through 201 d as to minimize the power of the signal component inthe lower sideband (step St31). With this, the imbalance compensationdevice 4 can minimize the skew between the in-phase component and thequadrature-phase component.

The power measuring units 42 a and 42 b then measure the power of thesignal component in the lower sideband extracted by the BPFs 41 a and 41b (step St32). The measured power is reported to the parameter adjustingunit 43, and is stored into a storage such as a memory in the parameteradjusting unit 43.

The parameter adjusting unit 43 then changes the quadrature deviationset value (step St33). For example, the parameter adjusting unit 43increases the set value by a predetermined value. The parameteradjusting unit 43 then determines whether the set value is within apredetermined adjustment range (step St34). The set value adjustmentrange is determined from the functions of the quadrature deviationadjusting units 202 a through 202 d, for example.

If the set value is within the adjustment range (Yes in step St34), theprocedures in step St32 and the steps that follow are again carried out.If the set value is outside the adjustment range (No in step St34), theparameter adjusting unit 43 sets such a set value in the quadraturedeviation adjusting units 202 a through 202 d as to minimize the powerof the signal component in the lower sideband (step St35). With this,the imbalance compensation device 4 can minimize the quadraturedeviation between the in-phase component and the quadrature-phasecomponent.

In this manner, the IQ imbalance compensation process is performed. Animbalance compensation method based on this compensation process canachieve the same effects as those of the imbalance compensation device4.

In this example, the BPFs 41 a and 41 b extract the crosstalk componentfrom the test signal Sr. However, the BPFs 41 a and 41 b do notnecessarily extract the crosstalk component, but may extract a signalcomponent of the source of the crosstalk component. For example, in acase where the test signal Sr has the spectrum indicated by referencesign G2 in FIG. 1, the BPFs 41 a and 41 b extract the signal componentin the upper sideband Sa. In a case where the test signal Sr has thespectrum indicated by reference sign G4 in FIG. 1, the BPFs 41 a and 41b extract the signal component in the lower sideband Sb.

That is, the BPFs 41 a and 41 b extract the signal component in theupper sideband Sa or the signal component in the lower sideband Sb,whichever has the greater mean power value. Where the crosstalkcomponent is larger, the extracted signal component has a smaller power.Therefore, the parameter adjusting unit 43 adjusts the respective setvalues of the amplitude adjusting units 200 a through 200 d, thedeskewing units 201 a through 201 d, and the quadrature deviationadjusting units 202 a through 202 d so that the power measured by thepower measuring units 42 a and 42 b is maximized. The compensationprocess in this case is described below.

FIG. 12 is a flowchart showing another example of the IQ imbalancecompensation process in the reception device 2. In FIG. 12, the sameprocedures as those shown in FIG. 11 are denoted by the same referencesigns as those used in FIG. 11, and explanation thereof will not berepeated.

The BPFs 41 a and 41 b extract the signal component in the uppersideband (USB component) from the signals Sx and Sy obtained from thetest signal Sr (step St22 a). As a result, the signal component in thesignal band of the test signal Sr is extracted. In a case where the SSBsignal generated in step St21 has its signal band in the lower sideband,the BPFs 41 a and 41 b extract the signal component in the lowersideband (LSB component) from the test signal Sr.

The power measuring units 42 a and 42 b measure the power of the signalcomponent in the upper sideband (USB component) (steps St24 a, St28 a,and St32 a). The parameter adjusting unit 43 sets the respective setvalues so that the measured power is maximized (steps St27 a, St31 a,and St35 a).

Thus, the imbalance compensation device 4 can minimize the powerdifference, the skew, and the quadrature deviation between the in-phasecomponent and the quadrature-phase component, by minimizing thecrosstalk component.

An IQ imbalance in the transmission device 1 can also be compensated forby an imbalance compensation device provided in the reception device 2.

FIG. 13 is a configuration diagram showing an example of a transmissionsystem that compensates for the IQ imbalance in a transmission device 1a. The transmission system includes the transmission device 1 a and areception device 2 a that are connected to each other by a transmissionpath 9 formed with an optical fiber. In FIG. 13, the same components asthose shown in FIGS. 6 and 9 are denoted by the same reference signs asthose used in FIGS. 6 and 9, and explanation thereof will not berepeated.

The transmission device 1 a includes transmission processing circuits 10a and 10 b, a filtering unit 18, a signal multiplexing unit 17, a DAconverting unit 12, and an optical modulating unit 19. The receptiondevice 2 a includes a coherent receiver 29, an AD converting unit 22, awavelength dispersion compensating circuit (a chromatic dispersioncompensator (CDC)) 27, a test signal acquiring unit 28, a receptionprocessing circuit 20, a BPF 41, a power measuring unit 42, and aparameter adjusting unit 43 a.

The transmission device 1 a selects a test signal or a client signal,and transmits the selected signal to the transmission path 9. Thetransmission processing circuits 10 a and 10 b each have the sameconfiguration as the above described transmission processing circuit 10.The transmission processing circuit 10 a performs a test signaltransmission process, and the transmission processing circuit 10 bperforms a client signal transmission process.

The test signal output from the transmission processing circuit 10 a isinput to the filtering unit 18. The filtering unit 18 includes the abovedescribed BPFs 18 a through 18 d. The test signal is converted into anSSB signal for each kind of polarization by the filtering unit 18, andis input to the signal multiplexing unit 17. It should be noted that thetest signal should be converted into a signal having a differencebetween the mean power value of the signal component in the uppersideband and the mean power value of the signal component in the lowersideband.

The client signal output from the transmission processing circuit 10 bis input to the signal multiplexing unit 17. It should be noted that theclient signal is a double-sideband (DSB) signal.

The signal multiplexing unit 17 performs time-multiplexing on testsignals and client signals, and outputs the resultant signals to the DAconverting unit 12. The test signals and the client signals arealternately output on the time axis, for example, as indicated byreference sign G6. The test signals contain data in a predeterminedpattern, for example, and are output in a training period during whichthe transmission characteristics are adjusted. It should be noted thatthe test signals are an example of a signal containing an in-phasecomponent and a quadrature-phase component.

The DA converting unit 12 includes the above described DACs 12 a through12 d. A test signal or a client signal converted into a digital signalby the DA converting unit 12 is modulated by the optical modulating unit19, and is transmitted to the transmission path 9.

The reception device 2 a receives a test signal or a client signal fromthe transmission device 1 a via the transmission path 9. The test signalor the client signal is input to the coherent receiver 29. The testsignal or the client signal output from the coherent receiver 29 isinput to the AD converting unit 22. The AD converting unit 22 includesthe above described ADCs 22 a through 22 d. The test signal or theclient signal converted into a digital signal by the ADCs 22 a through22 d is input to the CDC 27.

The CDC 27 compensates for wavelength dispersion caused in a test signalor a client signal in the transmission path 9. The test signal or theclient signal subjected to the wavelength dispersion compensation isinput to the test signal acquiring unit 28. It should be noted that theCDC 27 is formed with a circuit such as an FPGA.

The test signal acquiring unit 28 identifies a test signal from thepredetermined pattern, for example, and outputs the test signal to theBPF 41. The test signal acquiring unit 28 also outputs a client signalto the reception processing circuit 20. It should be noted that the testsignal acquiring unit 28 is formed with a circuit such as an FPGA.

The BPF 41 is an example of the extracting unit, and extracts the signalcomponent in the upper sideband or the signal component in the lowersideband from the test signal input from the test signal acquiring unit28. In a case where the test signal has its signal band in the uppersideband, the BPF 41 extracts the signal component in the lowersideband. In a case where the test signal has its signal band in thelower sideband, the BPF 41 extracts the signal component in the uppersideband. By doing so, the BPF 41 extracts the crosstalk componentgenerated due to an IQ imbalance from the test signal. It should benoted that the BPF 41 is an electrical filter.

The power measuring unit 42 is an example of the measuring unit, andmeasures the power of the signal component extracted by the BPF 41. Thepower measuring unit 42 notifies the parameter adjusting unit 43 a ofthe measured power.

The parameter adjusting unit 43 a is an example of the adjusting unit,and, in accordance with the power measured by the power measuring unit42, adjusts the parameters related to the IQ imbalance in thetransmission device 1 a. More specifically, in accordance with thepower, the parameter adjusting unit 43 a adjusts the respective setvalues of the amplitude adjusting units 102 a through 102 d and thedeskewing units 103 a through 103 d of the transmission processingcircuits 10 a and 10 b, and the phase shifters 16 a and 16 b of theoptical modulating unit 19. The respective setting values are an exampleof the parameters related to the IQ imbalance, and are transmitted fromthe reception device 2 a to the transmission device 1 a via a local areanetwork (LAN) or the like.

With this transmission system, it is possible to readily compensate forthe IQ imbalance in the transmission device 1 a in accordance with atest signal in a training period. In this example, each test signal isan SSB signal. However, a test signal is not limited to any particularsignal, as long as it is a signal having a difference between the meanpower value of the signal component in the upper sideband and the meanpower value of the signal component in the lower sideband.

Also, a test signal may be periodically switched between a signal havingits signal band in the upper sideband (this signal will be hereinafterreferred to as the “USB signal”) and a signal having its signal band inthe lower sideband (this signal will be hereinafter referred to as the“LSB signal”). With this, the powers of test signals are averagedtimewise between the upper sideband and the lower sideband, and thus,the above mentioned respective set values are adjusted with highprecision.

FIG. 14 is a configuration diagram showing another example of atransmission system that compensates for the IQ imbalance in atransmission device 1 b. In FIG. 14, the same components as those shownin FIG. 13 are denoted by the same reference signs as those used in FIG.13, and explanation thereof will not be repeated.

The transmission device 1 b includes transmission processing circuits 10a and 10 b, filtering units 180 and 181, a switching circuit 182, asignal multiplexing unit 17, a DA converting unit 12, and an opticalmodulating unit 19. A reception device 2 b includes a coherent receiver29, an AD converting unit 22, a CDC 27, a test signal acquiring unit 28,a reception processing circuit 20, BPFs 411 and 412, a switching circuit44, a power measuring unit 42, and a parameter adjusting unit 43 a.

A test signal output from the transmission processing circuit 10 a isdivided and input to the two filtering units 180 and 181. The filteringunits 180 and 181 are equivalent to the above described BPFs 18 athrough 18 d, and extract the signal component in the upper sideband orthe lower sideband so that the test signal turns into an SSB signal. Thefiltering unit 180 converts the test signal into a USB signal, and thefiltering unit 181 converts the test signal into an LSB signal. Therespective test signals converted into the USB signal and the LSB signalare input to the switching circuit 182.

The switching circuit 182 selects the USB signal or the LSB signal inaccordance with a select signal SELa, and outputs the selected signal tothe signal multiplexing unit 17. The select signal SELa is controlled bya control circuit (not shown) so that USB signals and LSB signals arealternately output at regular intervals. Accordingly, test signals thatare USB signals, and test signals that are LSB signals are alternatelytransmitted to the reception device 2 b at regular intervals.

In the reception device 2 b, a test signal output from the test signalacquiring unit 28 is divided and input to the two BPFs 411 and 412. TheBPF 411 extracts the signal component in the lower sideband from thetest signal, and the BPF 412 extracts the signal component in the uppersideband from the test signal. Therefore, when the test signal is a USBsignal, the crosstalk component of the test signal is input from the BPF411 to the switching circuit 44. When the test signal is an LSB signal,the crosstalk component of the test signal is input from the BPF 412 tothe switching circuit 44.

In accordance with a select signal SELb, the switching circuit 182selects the signal from the BPF 411 or the signal from the BPF 412, andoutputs the selected signal to the power measuring unit 42. The selectsignal SELb is synchronized with the select signal SELa of thetransmission device 1 b, and is controlled by a control circuit (notshown) so that signals from the BPF 411 and signals from the BPF 412 arealternately output at regular intervals. Thus, the crosstalk componentsof test signals that are USB signals, and the crosstalk components oftest signals that are LSB signals are alternately output to the powermeasuring unit 42 at regular intervals.

In this example, the powers of test signals are averaged timewisebetween the upper sideband and the lower sideband. Thus, the abovementioned respective set values are adjusted with high precision in thetransmission device 1 b. In this example, each test signal is an SSBsignal. However, each test signal is not limited to any particularsignal, as long as the signal has a difference between the mean powervalue of the signal component in the upper sideband and the mean powervalue of the signal component in the lower sideband.

Alternatively, a test signal may be an SSB signal only during theadjustment period for the above mentioned set values, because the testsignal is to be used for purposes other than the adjustment of the setvalues related to the IQ imbalance, as will be described below.

FIG. 15 is a configuration diagram showing another example of atransmission system that compensates for the IQ imbalance in atransmission device 1 c. In FIG. 15, the same components as those shownin FIG. 13 are denoted by the same reference signs as those used in FIG.13, and explanation thereof will not be repeated.

The transmission device 1 c includes transmission processing circuits 10a and 10 b, a switching circuit 182 a, a signal multiplexing unit 17, aDA converting unit 12, and an optical modulating unit 19. A receptiondevice 2 c includes a coherent receiver 29, an AD converting unit 22, aCDC 27, a test signal acquiring unit 28, a reception processing circuit20, a BPF 41, a power measuring unit 42, a parameter adjusting unit 43a, and a timing control unit 45.

A test signal output from the transmission processing circuit 10 a isdivided and input to a filtering unit 18 and the switching circuit 182a. The filtering unit 18 includes the above described BPFs 18 a through18 d. The test signal is converted into an SSB signal for each kind ofpolarization by the filtering unit 18, for example, and is then input tothe signal multiplexing unit 17. Here, the test signal should beconverted into a signal having a difference between the mean power valueof the signal component in the upper sideband and the mean power valueof the signal component in the lower sideband.

The test signal as an SSB signal is input from the filtering unit 18 tothe switching circuit 182 a, and the test signal as a double sideband(DSB) signal is input from the transmission processing circuit 10 a tothe switching circuit 182 a. In accordance with a select signal SELc,the switching circuit 182 a selects the SSB signal or the DSB signal,and outputs the selected signal to the signal multiplexing unit 17. Theselect signal SELc is controlled by the timing control unit 45 of thereception device 2 c so that a USB signal is output at a desired time.

During a training period, the transmission device 1 c normally transmitstest signals that are DSB signals. However, in a case where an IQimbalance compensation process is to be performed, the transmissiondevice 1 c transmits test signals that are SSB signals.

In the reception device 2 c, the timing control unit 45 notifies theswitching circuit 182 a in the transmission device 1 c and the parameteradjusting unit 43 a of the time to perform a compensation process on theIQ imbalance in the transmission device 1 c. Accordingly, the parameteradjusting unit 43 can adjust the above mentioned respective set valuesin accordance with the time at which an SSB signal is output from theswitching circuit 182 a. It should be noted that the timing control unit45 is formed with a circuit such as an FPGA.

FIG. 16 is a flowchart showing an example of a control operation in thetransmission system of this example. The transmission device 1 ctransmits a test signal that is a DSB signal (step St41). The timingcontrol unit 45 of the reception device 2 c then determines whether thetime for IQ imbalance compensation has come, in accordance with a timeror the like (step St42). If the time for compensation has not come yet(No in step St42), the procedures in steps St41 and St42 are againcarried out.

If the time for compensation has come (Yes in step St42), the timingcontrol unit 45 switches the switching circuit 182 a to the selectsignal SELc (step St43). The transmission device 1 c then transmits atest signal that is an SSB signal (step St44). The timing control unit45 then instructs the parameter adjusting unit 43 to adjust the abovementioned respective set values (step St45).

The transmission system operates in this manner. According to thisexample, IQ imbalance compensation can be performed at a desired time.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various change, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An imbalance compensation device that compensatesfor an imbalance between an in-phase component and a quadrature-phasecomponent of a signal, the imbalance compensation device comprising: anextracting unit that extracts a signal component in an upper sideband ora signal component in a lower sideband from the signal; a measuring unitthat measures power of the signal component in the upper sideband or thesignal component in the lower sideband extracted by the extracting unit;and an adjusting unit that adjusts a parameter related to the imbalance,in accordance with the power measured by the measuring unit.
 2. Theimbalance compensation device of claim 1, wherein the signal is a singlesideband signal.
 3. The imbalance compensation device of claim 1,wherein the extracting unit extracts the signal component in the uppersideband or the signal component in the lower sideband, whichever hassmaller power, and the adjusting unit adjusts the parameter to minimizethe power measured by the measuring unit.
 4. The imbalance compensationdevice of claim 1, wherein the extracting unit extracts the signalcomponent in the upper sideband or the signal component in the lowersideband, whichever has larger power, and the adjusting unit adjusts theparameter to maximize the power measured by the measuring unit.
 5. Atransmission device that transmits a signal containing an in-phasecomponent and a quadrature-phase component, the transmission devicecomprising: a converting unit that converts the signal to cause adifference between power of a signal component in an upper sideband ofthe signal and power of a signal component in a lower sideband of thesignal; an extracting unit that extracts the signal component in theupper sideband or the signal component in the lower sideband from thesignal converted by the converting unit; a measuring unit that measuresthe power of the signal component in the upper sideband or the signalcomponent in the lower sideband extracted by the extracting unit; and anadjusting unit that adjusts a parameter related to an imbalance betweenthe in-phase component and the quadrature-phase component, in accordancewith the power measured by the measuring unit.
 6. The transmissiondevice of claim 5, wherein the signal is a single sideband signal. 7.The transmission device of claim 5, wherein the extracting unit extractsthe signal component in the upper sideband or the signal component inthe lower sideband, whichever has smaller power, and the adjusting unitadjusts the parameter to minimize the power measured by the measuringunit.
 8. The transmission device of claim 5, wherein the extracting unitextracts the signal component in the upper sideband or the signalcomponent in the lower sideband, whichever has larger power, and theadjusting unit adjusts the parameter to maximize the power measured bythe measuring unit.
 9. A reception device that receives a signalcontaining an in-phase component and a quadrature-phase component, thereception device comprising: an extracting unit that extracts a signalcomponent in an upper sideband or a signal component in a lower sidebandfrom the signal; a measuring unit that measures power of the signalcomponent in the upper sideband or the signal component in the lowersideband extracted by the extracting unit; and an adjusting unit thatadjusts a parameter related to an imbalance between the in-phasecomponent and the quadrature-phase component, in accordance with thepower measured by the measuring unit.
 10. The reception device of claim9, wherein the signal is a single sideband signal.
 11. The receptiondevice of claim 9, wherein the extracting unit extracts the signalcomponent in the upper sideband or the signal component in the lowersideband, whichever has smaller power, and the adjusting unit adjuststhe parameter to minimize the power measured by the measuring unit. 12.The reception device of claim 9, wherein the extracting unit extractsthe signal component in the upper sideband or the signal component inthe lower sideband, whichever has larger power, and the adjusting unitadjusts the parameter to maximize the power measured by the measuringunit.
 13. The reception device of claim 9, further comprising aconverting unit that converts the signal to cause a difference betweenthe power of the signal component in the upper sideband of the signaland the power of the signal component in the lower sideband of thesignal, wherein the extracting unit extracts the signal component in theupper sideband or the signal component in the lower sideband from thesignal converted by the converting unit.
 14. A reception device thatreceives a signal containing an in-phase component and aquadrature-phase component from a transmission device, the receptiondevice comprising: an extracting unit that extracts a signal componentin an upper sideband or a signal component in a lower sideband from thesignal; a measuring unit that measures power of the signal component inthe upper sideband or the signal component in the lower sidebandextracted by the extracting unit; and an adjusting unit that adjusts aparameter related to an imbalance between the in-phase component and thequadrature-phase component for the transmission device, in accordancewith the power measured by the measuring unit.
 15. The reception deviceof claim 14, wherein the extracting unit extracts the signal componentin the upper sideband or the signal component in the lower sideband,whichever has smaller power, and the adjusting unit adjusts theparameter to minimize the power measured by the measuring unit.
 16. Thereception device of claim 14, wherein the extracting unit extracts thesignal component in the upper sideband or the signal component in thelower sideband, whichever has larger power, and the adjusting unitadjusts the parameter to maximize the power measured by the measuringunit.
 17. An imbalance compensation method of compensating for animbalance between an in-phase component and a quadrature-phase componentof a signal, the imbalance compensation method comprising: extracting asignal component in an upper sideband or a signal component in a lowersideband from the signal; measuring power of an extracted signalcomponent in the upper sideband or an extracted signal component in thelower sideband; and adjusting a parameter related to the imbalance, inaccordance with measured power.
 18. The imbalance compensation method ofclaim 17, wherein the signal is a single sideband signal.
 19. Theimbalance compensation device of claim 17, wherein the signal componentin the upper sideband or the signal component in the lower sideband,whichever has smaller power, is extracted, and the parameter is adjustedto minimize the measured power.
 20. The imbalance compensation device ofclaim 17, wherein the signal component in the upper sideband or thesignal component in the lower sideband, whichever has larger power, isextracted, and the parameter is adjusted to maximize the measured power.